a) Field of the Invention
The present invention relates to a method for injecting and dissolving a gas, such as air, into a liquid that is preferably water, while this liquid is being pumped by a centrifugal pump, that is preferably of the rotary disc type.
The invention also relates to a centrifugal liquid pump, preferably of the rotary disc type, incorporating a gas injection assembly.
b) Brief Description of the Prior Art
In the flotation processes that are presently used for "clarifying" or otherwise treating waste water it is of common practice to recycle part of the clarified water. Usually, the clarified water is pumped at the bottom of the flotation tank of the clarifier or at the outlet of the same and injected into the waste water to be treated just before it enters the clarifier.
It is also of common practise to inject air into the waste water that enters the clarifier, in such a manner as to generate a multitude of very small bubbles which "catch" the solids in suspension in the waste water and thus favorize flotation of the same. Such an injection can be made either directly into the waste water fed to the clarifier, just before it enters the same, or preferably into the clarified water that is recycled prior to its injection into the waste water. In both cases, the injection is preferably made under pressure so as to dissolve as much air as possible in the water.
In order to recycle a sufficient amount of clarified water and simultaneously allow dissolution therein of a sufficient amount of air to generate a multitude of micro bubbles of 150.mu.or less as soon as the pressure is released, the pump must ideally generate a pressure of 80 to 120 lbs. Of course, it must also have ideally a low energy consumption (expressed in m.sup.3 per horse power).
To meet these goals, use has been made so far of centrifugal multistage pumps with bladed impellers that can build up pressure up to 200 lbs. However, these pumps have a low flow rate.
It has also been suggested to use rotary disc pumps comprising a plurality of closely spaced apart discs rotatably mounted within a casing (see for example U.S. Pat. Nos. 4,335,996; 4,514,139; 4,768,920 and 4,773,819). In this particular case, the pumping effect is obtained by frictional and shear forces developed between the rotating discs and the fluid. To improve such an effect, it has also been suggested to provide radial straight ribs on each disc (see U.S. Pat. No. 4,940,385).
Rotary disc pumps are interesting in that, thanks to their structure, they can easily handle a fluid such as waste water, which may contain solids in suspension. However, they are really effective only when the pressure to be built up is lower than 50 lbs. Moreover, they are known to be energy consuming (maximum of 1 m.sup.3 /HP).
To provide the required dissolution of air in the recycled water (or in the waste water fed into the clarifier), it is of common practice to provide an air inlet in a venturi located upstream the pump, so as to suck air with and into the water and to compress with the same within the pump (see, for example, Canadian patent No. 1,016,408, even if it is directed to another application).
In this very specific field, it has also been suggested to inject air directly within the casing of the pump, either through conducts made in the blades of the impeller and opening at the outer ends of these blades (see U.S. Pat. No. 3,485,484) or through stationary pins extending in the casing of the pump, the blades of the rotor then being split at a given radial distance from their rotation axis not to interfere with the pins (see U.S. Pat. No. 4,744,722). In both of these cases, the casing and/or the impeller or rotor is/are of very specific structure, thereby making the pump rather expensive and its structural components sometimes difficult to repair and/or easily interchange.
It is known that with bladed impeller multistage pumps capable of building up high pressure it is possible to mix up to 20% per volume of air in the water flow. With the existing rotary disc pumps which cannot build up high pressure, one may mix up to 7% per volume of air only, and only if the discs are close to each other and rotate at a speed of 1700 to 2100 rpm. However, in practice, from 10 to 15% per volume of air are required to make the waste water treatment efficient in the clarifier.